Assembling substrates by molecular adhesion (direct wafer bonding) is a well-known technique that finds applications in the fields of microelectronics, optoelectronics, electromechanical microsystems, for example, for manufacturing silicon-on-insulator substrates, multiple-junction photovoltaic cells and the production of 3D structures.
According to this technique, two substrates are put in close contact so as to bring their surfaces sufficiently close to each other so that they establish between them atomic and/or molecular bonds (hydroxyl or covalent bonds). The presence of water at the assembly interface assists the creation of such bonds. In this way, adhesion forces are created between the two surfaces of the substrates in contact, without using any intermediate adhesion layer, such as a layer of glue or a polymer.
The assembly obtained is then generally subjected to heat treatment at a temperature that may vary between 50° C. and 1200° C., depending on the nature of the substrates and the application envisaged, so as to reinforce the adhesion.
Assembly by molecular adhesion causes, in some cases, the appearance of defects, referred to as “bonding defects,” at the bonding interface. It may be a defect of the “bubble” type (bonding voids). The bonding defects may result from the trapping and accumulation of gaseous species between the surfaces of the assembled substrates. These species may correspond to the species adsorbed on the surfaces of the substrates when they are prepared before assembly; they may correspond to residues of chemical reactions, in particular, the chemical reaction of water, which occurs when the substrates are put in close contact or during the bonding reinforcement annealing. A description of the chemical phenomena that occurs during assembly by molecular adhesion is, for example, described in the article “Hydrophilic Low-Temperature Direct Wafer Bonding” by C. Ventosa et al., Journal of Applied Physics 104, 123524 (2008) or in the article “A Review of Hydrophilic Silicon Wafer Bonding” by V. Masteika et al., ECS Journal of Solid State Science and Technology, 3(4) Q42-Q54 (2014).
The presence of bonding defects at the assembly interface is highly detrimental to the quality of the structures produced. For example, when the assembly step is followed by a step of thinning two substrates in order to form a layer, by grinding or in accordance with SMART CUT® technology, the absence of adhesion between the two surfaces at a bonding defect may lead to local tearing away of the layer at this point. In the case of a 3D integration of components, a bonding defect prevents the components formed on each of the substrates being put in electrical contact, which makes these components non-functional.
One solution envisaged for reducing assembly defects and, in particular, bonding defects, is proposed in U.S. Publication No. 2013/0139946, the contents of which are hereby incorporated herein in their entirety by this reference. This document discloses a method for assembly by molecular adhesion comprising the circulation of a flow of gas over the surfaces of the substrate before they are assembled.
This method discharges the water molecules desorbed on the surfaces by the circulation of the gaseous flow, outside the bonding chamber. And, according to this document, by preventing the saturation of the atmosphere of the chamber with water, the method keeps the quality constant from one assembly to the next.
However, applying this method is tricky and may, for example, depending on the nature of the substrates assembled and after the reinforcement treatment, lead to an insufficient degree of adhesion between the substrates or to the presence of residual bonding defects. It should be noted, in particular, that a gas flow is an important carrier of particulate contamination to which molecular adhesion is highly sensitive, since the particles may give rise to bonding defects.